A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Photolithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. At present, no alternative technology seems to provide the desired pattern architecture with similar accuracy, speed, and economic productivity. However, as the dimensions of features made using photolithography become smaller, photolithography is becoming one of the most, if not the most, critical gating factors for enabling miniature IC or other devices and/or structures to be manufactured on a truly massive scale.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size of a feature arranged in an array with a 1:1 duty cycle (i.e., equal lines and spaces or holes with size equal to half the pitch). Thus, in the context of an array of features characterized by a certain pitch at which the features are spaced in the array, the critical dimension CD in equation (1) represents the value of half of a minimum pitch that can be printed, referred to hereinafter as the “half-pitch.”
It follows from equation (1) that a reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
Current resolution enhancement techniques that have been extensively used in lithography to lower the Rayleigh constant k1, thereby improving the pattern resolution, include the use of, for example, attenuated or alternating phase shift masks and/or sub-resolution assist features (SRAF). These resolution enhancement techniques are of particular importance for lithographic printing and processing of contact holes or vias which define connections between wiring levels in an IC device, because contact holes have, compared to other IC features, a relatively small area. Contact holes may be printed, for example, using conventional on-axis illumination in combination with a phase shift mask and a positive resist.
However, the use of attenuated or alternating phase shift masks and/or sub-resolution assist features may not be feasible to pattern small features and contact holes below about 85 nm (at λ=193 nm, NAPS=0.93, and k1=0.4). These techniques have limited capabilities and may not provide sufficient process latitude (i.e., the combined usable depth of focus and allowable variance of exposure dose for a given tolerance in the critical dimension) to print half-pitches below a CD obtainable when operating at k1=0.4.